Field of the Invention
The present invention relates to a liquid crystal display and more particularly to a multiple view liquid crystal display capable of displaying a plurality of images in different directions from each other.
Description of the Background Art
A multiple view liquid crystal display (a plural screen liquid crystal display) using a parallax barrier is developed as one of value added technologies of a liquid crystal display. In particular, there is often proposed a two-screen liquid crystal display for separating and displaying two different screens at the same time.
A multiple view liquid crystal display of a parallax barrier type has a structure including a liquid crystal display panel (hereinafter referred to as a “liquid crystal panel”) in which pixels for displaying images are mixed and arranged in accordance with a predetermined rule, and a light shielding layer which is provided on a front side (a visual recognition side) and is referred to as a parallax barrier. The parallax barrier is disposed to shield light emitted from each pixel of the liquid crystal panel in a specific direction. Consequently, the light emitted from the liquid crystal panel is separated in a plurality of directions, and a plurality of images displayed by the liquid crystal panel is displayed in different directions from each other.
The multiple view liquid crystal display of the parallax barrier type has a problem of a crosstalk in which a part of images to be displayed in a certain direction are observed with leakage in images displayed in the other direction.
For example, in the case in which the crosstalk occurs in the two-screen liquid crystal display of the parallax barrier type for transversely dividing and displaying two images, the image (the left image) to be displayed as viewed from the left side to the screen and the image (the right image) to be displayed as viewed from the right side thereto are seen with overlapping. The crosstalk occurs when ranges of viewing angles (visual field ranges) of the respective images overlap with each other. For this reason, the crosstalk tends to occur in the vicinity of a boundary between the visual field ranges of the respective images. In other words, in the two-screen liquid crystal display, the crosstalk tends to occur as viewed from the front of the screen, that is, a boundary between the visual field range of the right image and the visual field range of the left image. When an image having much black display (low luminance display) is displayed, particularly, leakage from the other image is easily recognized visually even though it is very small. Therefore, image quality is greatly influenced.
In general, the liquid crystal panel has a structure including a first substrate provided with a pixel electrode and a switching element, a signal line or the like for supplying a pixel signal thereto, a second substrate provided with a black matrix for defining each pixel region and a color filter (CF), and a liquid crystal interposed therebetween. In the multiple view liquid crystal display of the parallax barrier type, the black matrix for defining the pixel region is formed on an opposed surface to the first substrate in the second substrate, and the parallax barrier is formed on an opposite surface (on the visual recognition side). Consequently, a gap corresponding to a thickness of the second substrate is present between the parallax barrier and the black matrix. A size of the gap is an element for determining a direction and an area of the visual field range of each of the images to be simultaneously displayed together with a size of an opening portion of the parallax barrier, a pitch of a pixel or the like.
In the multiple view liquid crystal display of the parallax barrier type, a phenomenon referred to as “reverse viewing” is caused by the presence of the gap between the parallax barrier and the black matrix. In the reverse viewing, an image to be displayed in a reverse direction is seen when the screen is seen in a direction deviating largely from the front of the screen. For example, when an observer moves rightward from the front of the screen with respect to the two-screen liquid crystal display, the right image is first seen. When the observer further continues to move rightward, a left image is seen within some range. This range is generated because another pixel is seen adjacently to a pixel to be originally seen through the opening portion of the parallax barrier.
In other words, in the two-screen liquid crystal display of the parallax barrier type, the visual field range for the left image due to the reverse viewing phenomenon is present on an outside of the visual field range for the right image, and the visual field range for the right image due to the reverse viewing phenomenon is present on the outside of the visual field range for the left image. For this reason, the crosstalk of the right image and the left image tends to occur in the vicinity of outer ends of the respective visual field ranges for the right image and the left image in addition to the vicinity of the front of the screen. A crosstalk occurring in the vicinity of the front of the screen will be referred to as a “front crosstalk” and a crosstalk caused by the reverse viewing phenomenon occurring in the vicinity of the outer end of the visual field range for each of the images will be referred to as a “reverse viewing crosstalk”.
The front crosstalk and the reverse viewing crosstalk also become a problem in an optional multiple view liquid crystal display of a parallax barrier type in addition to the two-screen liquid crystal display.
In recent years, in a general liquid crystal display, there is expanded application of an In Plane Switching mode (including an FFS (Fringe Field Switching) mode and referred to as a “horizontal electric field mode”) which is a method of mainly applying an electric field in a horizontal direction with respect to a substrate surface to drive a liquid crystal in place of a TN (Twisted Nematic) mode in the related art. A liquid crystal display in the horizontal electric field mode has a wide visual field range. By application to a multiple view liquid crystal display assuming observation in different directions, therefore, it is possible to expect considerable enhancement in display grade of the multiple view liquid crystal display. A structure of the multiple view liquid crystal display applying the horizontal electric field mode is disclosed in the following Japanese Patent Application Laid-Open No. 2008-064918, for example.
A crosstalk rate Rxt indicative of an intensity of a crosstalk occurring in a two-screen liquid crystal display can be expressed as follows:Rxt=(Lwb−Lbb)/MIN(Lwb,Lbb)  Equation (1)In the equation (1), Lwb represents a luminance (a black luminance) on an observation side when black display is carried out on the observation side and white display is carried out on the other side (which will be hereinafter referred to as a “black luminance in white-black display”), and Lbb represents a luminance (a black luminance) on the observation side when the black display is carried out on the observation side and the other side (which will be hereinafter referred to as a “black luminance in black-black display”). “MIN(Lwb,Lbb)” is a function taking a smaller one of values of Lwb and Lbb.
The equation (1) indicates a rate of light leaking from the other side carrying out the white display toward a visual field range on the observation side carrying out the black display. In order to decrease the crosstalk rate, two types of methods, that is, reduction in the black luminance Lwb in the white-black display and increase in the black luminance Lbb in the black-black display are effective. The method of reducing the black luminance Lwb in the white-black display serves to suppress light leakage from the white display side, thereby decreasing the crosstalk itself. On the other hand, the method of increasing the black luminance Lbb in the black-black display serves to increase the light leakage in the black display, thereby making the crosstalk unremarkable (reducing visibility).
A relationship between the crosstalk and the liquid crystal display mode will be considered. The TN mode in the related art is a normally white (NW) mode, while the horizontal electric field mode is a normally black (NB) mode. For this reason, the horizontal electric field mode has a higher contrast ratio (CR) than the TN mode, that is, has a lower black luminance.
The present inventor found a new problem. More specifically, if a liquid crystal panel in the horizontal electric field mode is used to constitute the multiple view liquid crystal display of the parallax barrier type, the crosstalk is deteriorated more greatly due to a low black luminance based on a high contrast ratio as compared with the case in which the TN mode is used. If the black luminance is increased in order to suppress the crosstalk, however, it is impossible to make the best of an advantage of the horizontal electric field mode, that is, a high contrast ratio. This problem is also caused in the multiple view liquid crystal display using the horizontal electric field mode disclosed in the Japanese Patent Application Laid-Open No. 2008-064918.